(1) Field of the Invention
The present invention relates to a photosensitive plate used for the production of a lithographic plate to be used in the dry lithography, i.e. a lithographic printing plate requiring no dampening solution.
(2) Description of the Prior Art
Various dry lithographic printing plates have been known in which a photosensitive layer and a silicone rubber layer are formed in sequence on a base. Such lithographic printing plates are disclosed, for example, in Japanese Patent Publication Nos. 23042/1969, 26923/1979 and U.S. Pat. No. 3,511,178, as well as in Japanese Patent Application (OPI) (the term "OPI" as used herein means as unexamined published patent application) No. 22781/1980, U.S. Pat. No. 4,358,522, B.P. 2,034,911 and U.S. Pat. No. 4,342,820. Among them, dry lithographic plates comprising the photodimerizable photosensitive resin disclosed in Japanese Patent Publication No. 22781/1980 are less liable to be influenced by oxygen during plate making and exposure steps than the dry photosensitive lithographic plates having photopolymerizable sensitive layer of the type shown in Japanese Patent Publication No. 26923/1979. Therefore, satisfactory printing is possible even when the plates have no substantial oxygen-impermeable overcoating layer for example the plates having no projection pattern or the substantially oxygen permeable thin coating layer on the silicone rubber layer to improve the vacuum contact of original film to the plates, or even when the original film is directly placed on the silicone rubber layer so as to gain the improved tone reproduction.
Further, the dry photosensitive lithographic plates containing the photodimerizable sensitive resin are superior to the plates containing the photopolymerizable photosensitive composition in that the former is less liable to be influenced by the exposure temperature in the plate-making and exposing steps and in that the sensitivity of the former does not vary with the retention time from the exposure to the development steps or the temperature in these steps and, therefore, stable plate making is possible when the former plates are used.
However, special treatments were necessitated to prepare the former plates. These treatments include, for example, the formation of an adhesive intermediate layer between the ink-repelling silicone rubber layer and the photodimerizable photosensitive layer; the incorporation of a silane coupling agent having an aminoalkyl group in a silicone rubber composition used for forming the silicone-rubber layer; the introduction of a hydroxyl group or the like reactive with the silicone rubber layer in the photodimerizable photosensitive material to improve the adhesion of these layers each other; and the heat treatment effected in the course of the drying and curing of the silicone rubber composition.
Among these processes, the process wherein the adhesive intermediate layer is formed between the photosensitive layer and silicone rubber layer so as to improve the adhesion is not satisfactory, since the developability of the obtained dry lithographic plate is deteriorated and the number of steps is increased because of the formation of the adhesive intermediate layer in the preparation thereof, though stable adhesive force is obtained by this process and the product can be stored for a long period of time (1 to 2 years) without causing any trouble. The process wherein the silane coupling agent having an aminoalkyl group is incorporated in the silicone rubber composition has defects in that when the obtained lithographic plate is left to stand for a period of as long as six months the adhesion is deteriorated and sufficient image-reproduction becomes impossible and that when the processed plate is left to stand for several days, the silicone rubber layer (non-image-forming area) falls off easily during the printing and it becomes difficult to obtain a sharp image printing, though it has been said that this process is preferred because the developability is improved remarkably and the step of forming the intermediate layer can be omitted. The process wherein a functional group reactive with the silicone rubber layer is introduced in the photodimerizable photosensitive material per se has problems in that the material cannot be synthesized easily and that the intended photodimerizable photosensitive material having the desired properties cannot be obtained easily.
Also proposed is a negative dry photosensitive lithographic printing plate of the type in which, as disclosed in Japanese Patent Application OPI Nos. 55-59466/1980 and 110249/1980, a photosensitive layer containing orthoquinone diazido compound is applied to an aluminum base and then a silicone rubber layer is formed on the photosensitive layer. According to Japanese Patent Application OPI No. 110249/1980, it is possible to obtain a negative dry photosensitive lithographic printing plate is obtained when the silicone rubber composition contains aminosilane expressed by the following formula. EQU R.sub.m R'.sub.n Si(O R") 4-m-n
where, R represents an alkyl group containing a non-substituted or mono-substituted amino group, R' and R" represent an alkyl group or an allyl group, m represents 1 or 2, and n represents 0 or 1, wherein the condition of m+n=1 or m+n=2 is met.
However, the addition of the aminosilane to the silicone rubber causes a problem in that the curing speed is significantly decreased so that the silicone rubber exhibits, when left for a long time, reduces the bonding power with the result that the bonding power is significantly reduced so as to deteriorate the image reproducibility undesirably.